The system libraries define a standard set of functions through which applications interact with the kernel, and which implement much of the operating-system functionality that does not need the full privileges of kernel code

Supports loading modules into memory and letting them talk to the rest of the kernel

Module loading is split into two separate sections:

Managing sections of module code in kernel memory

Handling symbols that modules are allowed to reference

The module requestor manages loading requested, but currently unloaded, modules; it also regularly queries the kernel to see whether a dynamically loaded module is still in use, and will unload it when it is no longer actively needed

1. Normal kernel code is nonpreemptible (until 2.4) – when a time interrupt is received while a process is executing a kernel system service routine, the kernel’s need_resched flag is set so that the scheduler will run once the system call has completed and control is about to be returned to user mode

2. The second technique applies to critical sections that occur in an interrupt service routines

– By using the processor’s interrupt control hardware to disable interrupts during a critical section, the kernel guarantees that it can proceed without the risk of concurrent access of shared data structures

To avoid performance penalties, Linux’s kernel uses a synchronization architecture that allows long critical sections to run without having interrupts disabled for the critical section’s entire duration

Interrupt service routines are separated into a top half and a bottom half.

The top half is a normal interrupt service routine, and runs with recursive interrupts disabled

The bottom half is run, with all interrupts enabled, by a miniature scheduler that ensures that bottom halves never interrupt themselves

This architecture is completed by a mechanism for disabling selected bottom halves while executing normal, foreground kernel code

The VM system maintains the address space visible to each process: It creates pages of virtual memory on demand, and manages the loading of those pages from disk or their swapping back out to disk as required

The VM manager maintains two separate views of a process’s address space:

A logical view describing instructions concerning the layout of the address space

The address space consists of a set of nonoverlapping regions, each representing a continuous, page-aligned subset of the address space

A physical view of each address space which is stored in the hardware page tables for the process

The Linux kernel reserves a constant, architecture-dependent region of the virtual address space of every process for its own internal use

This kernel virtual-memory area contains two regions:

A static area that contains page table references to every available physical page of memory in the system, so that there is a simple translation from physical to virtual addresses when running kernel code

The reminder of the reserved section is not reserved for any specific purpose; its page-table entries can be modified to point to any other areas of memory

Ext2fs uses a mechanism similar to that of BSD Fast File System (ffs) for locating data blocks belonging to a specific file

The main differences between ext2fs and ffs concern their disk allocation policies

In ffs, the disk is allocated to files in blocks of 8Kb, with blocks being subdivided into fragments of 1Kb to store small files or partially filled blocks at the end of a file

Ext2fs does not use fragments; it performs its allocations in smaller units

The default block size on ext2fs is 1Kb, although 2Kb and 4Kb blocks are also supported

Ext2fs uses allocation policies designed to place logically adjacent blocks of a file into physically adjacent blocks on disk, so that it can submit an I/O request for several disk blocks as a single operation

Like UNIX, Linux informs processes that an event has occurred via signals

There is a limited number of signals, and they cannot carry information: Only the fact that a signal occurred is available to a process

The Linux kernel does not use signals to communicate with processes with are running in kernel mode, rather, communication within the kernel is accomplished via scheduling states and wait.queue structures

The pipe mechanism allows a child process to inherit a communication channel to its parent, data written to one end of the pipe can be read a the other

Shared memory offers an extremely fast way of communicating; any data written by one process to a shared memory region can be read immediately by any other process that has mapped that region into its address space

To obtain synchronization, however, shared memory must be used in conjunction with another Interprocess-communication mechanism